The Global Challenge can only accept teams of seven. If someone pulls out, it is required that the person be replaced by the Team Captain via the ‘Manage teams’ page so the team is not disadvantaged. The replacement member will inherit the previous member's entered steps prior to the change (but thankfully not their shoes)!
If you don't have seven people yet, speak with more of your colleagues and encourage them to get involved. Don't forget; it's also possible to form teams of colleagues from different offices and countries as well.